Studies on aeration-I

25 Pages·2008·2.93 MB·English

Bull. Org. mond. Sant|
1952, 6, 73-97
Bull. WorldHlth Org.
STUDIES ON AERATION I
-
E. B. CHAIN, M.A., Ph.D., F.R.S.*
S. PALADINO, D. Ing.
D. S. CALLOW
F. UGOLINI, D. Ing.
J. VAN DER SLUIS
Internationai Research Centrefor Chemical Microbiology,
Istituto Superiore di Sanitd, Rome
A. FACTORS INFLUENCING AERATION EFFICIENCY
Aerationisessential forthegrowthofmanymicro-organisms ofscientific
andindustrialinterest and forthe production ofcertain oftheirmetabolites.
An efficient aeration technique is therefore one ofthe fundamental require-
ments of fermentation technology. Although many aeration devices have
been described and are in use, they have been designed mainly on an
empirical basis and few data have been published which would allow of
the quantitative evaluation oftheir respective efficiencies.
Efficient aeration depends upon bringing theliquid into intimate contact
with the largest obtainable surface of air. Oxygen would obviously be
much more efficacious than air, but is too expensive for most industrial
purposes. In order to bring the liquid into intimate contact with air it is
customary to introduce the air into the liquid by means of a sparger, the
function of which is supposed to be-but is not always-a preliminary
dispersion of the air stream into fine bubbles. Concurrently, the liquid is
mechanically agitated. This causes a further division of the air-bubbles
into still finer bubbles, and mixes them efficiently with all parts of the
liquid, thus facilitating the diffusion of oxygen and prolonging the phase
during which the air and liquid are in contact. In this type of aeration
power is consumed for (a) the production of air (the larger part of which
frequently passes through the liquid without being utilized), and (b) the
mechanical agitation of the liquid.
Clearly the most efficient aeration deviceis thatwhich provides maximal
aeration for the minimal power consumption. The purpose ofthe present
study was to compare on a quantitative basis the efficiency (i.e., oxygen
uptake per power absorbed per unit volume and time) ofdifferent aeration
systems. This investigation was of special interest in view of certain
*Scientific Director, International Research Centre for Chemical Microbiology, Istituto Superiore
di Sanita, Rome
129 -73 -
74 E. B. CHAIN AND OTHERS
observations made by the authors which had led to the development of a
novel aeration device, and to the necessity for comparing its efficiency
with that of standard systems.
Methods and Results
System usedfor oxygen absorption
For comparative aeration studies, the oxidation ofsulfite ions to sulfate
ions in aqueous solution under the catalytic action of copper ions was
chosen as a means of assessing the degree of aeration. The system, first
introduced by Cooper, Fernstrom & Miller,' is convenient because
(1) the rate of reaction between dissolved oxygen and sulfite is very
rapidinthepresence ofcopper, so thatthereis noappreciablebackpressure
FIG. 1. APPARATUS FOR MEASUREMENT OF OXYGEN UPTAKE - I
STUDIES ON AERATION - I 75
FIG. 2. APPARATUS FOR MEASUREMENT OF OXYGEN UPTAKE -11
of oxygen and the limiting factor of the reaction velocity is the rate of
diffusion of oxygen from the gas phase into the liquid, i.e., one of the
factors determining the aeration efficiency, and
(2) the rate ofreactionis, withinwidelimits (0.1-1 normal), independent
of the concentration of sulfite ions. The sulfite solution used was 0.5 N
to which copper sulfate was added to give a final concentration of
1/10,000 M CuS04,5H20.
The oxygen uptake was measured by following the disappearance of
sulfite ions by iodometric titration. During the period of one hour, after
time-intervals of 15 to 30 minutes depending on the degree of aeration,
2-ml samples of the sulfite solution were pipetted into 3 ml of 0.5 N
iodine solution and the excess iodine was titrated with 0.5 N thiosulfate.
All measurements were made in duplicate or triplicate. Reproducible
76 E. B. CHAIN AND OTHERS
results were obtained irrespective ofwhether or not the pipettes were rinsed
with nitrogen before the samples were taken. The rate of disappearance
of the sulfite ions remained constant for a period- of 1 to 1.5 hours; after
this the rate began to increase for reasons not properly understood. The
titration results for the absorption ofoxygen wereconfirmed by manometric
measurements in Warburg flasks and good agreement was obtained. All
measurements were made in a cylindrical glass vessel (fig. 1 and 2) with
a total capacity of 10 litres and a workingvolume of6 litres to give the ratio
height of liquid 1: 1
diameter of vessel
In some of the experiments baffle-plates were used. These were four
metal strips, 23 cm long and 1.5 cm wide, uniformly spaced in the reaction
vessel.
Measurement ofpower absorption
The power absorption for the mechanical agitation of the liquid was
measured by the conventional dynamometric methods. For power absorp-
tion of the order of magnitude of 0.6-6 watts, that is 0.1-1 watt per litre
of solution, the dynamometer illustrated in fig. 1 was used. A small motor
driving the stirrer was mounted on a ball-race allowing the motor to rotate
freely. A sector of a disc with a known diameter was rigidly fixed to the
motor-housing. It was connected by a thread to a load which could be
increased to a pointjust sufficient to prevent the motor from rotating. This
load was weighed on a balance and is in direct proportion to the work done
by the rotating system. A blank value for the power absorbed by the
motor rotating in air was obtained and deducted from the experimental
values. By a simple calculation the weights can be converted into any
suitable units ofwork. The authors haveexpressed the results in watt-hours
per%re
of solution.
For power absorption of the order of magnitude of 5-40 watts, i.e.,
0.83-6.7 watts per litre, the dynamometer illustrated in fig. 2 was used.
This was similar to the apparatus used by Cooper, Fernstrom & Miller.'
In this case the motor was fixed and the vessel freely rotating. The force
necessary to counterbalance the rotation of the vessel was again measured
by weights.
Comparisonofefficienciesofairspargerswithandwithoutmechanicalagitation
The form ofair spargers in practical use varies considerably; they may
be of circular, rectangular, or spiral shape, with holes of varying numbers
(from one to several hundred) and dimensions, and placed in different
positions (sometimes on the upper surface, sometimes on the lower surface)
on the spargers.
STUDIES ON AERATION - 1 77
Spargers of porous material, such as porous stone, carborundum,
stainless steel, or brass, are also employed. In the experiments reported
here, the aeration efficiency of different spargers with different air flows
(measured by rotameters a) was first studied without agitation ofthe liquid.
FIG. 3. TYPES OF AIR SPARGERS USED
Ringsporger Pbrm-hnte Su?gIe..rifwe
sparger sparger
/N t's.:.'-'i.".|ih'~~~~~~~~~~~...'..*..'.:......
The spargers used (see fig. 3) were (1) circular spargers of two diameter
dimensions, one slightly smaller than the diameter of the reaction vessel
and the other about one-third of the diameter of the reaction vessel; (2) a
sparger with a single orifice of 1-mm diameter; and (3) a cylindrical sparger
ofporous stone. As shown in table I, aeration with the circular and single-
orifice spargers was, poor in the absence of mechanical agitation, even
with high air flows (1 volume of air per volume of liquid per minute).
The aeration given by the smaller sparger was better than that given by
the larger sparger, while the aeration given by the single-orifice sparger
was better than that ofthe smaller ring sparger containing 27 or 90 holes,
probably owing to the higher air velocity at the single orifice. It should
be noted that in ring spargers the air is expelled only through some of
the holes and not equally through all of them. The aeration by means of
the porous-stone sparger, in which the air was divided into a large number
ofvery small bubbles, provedto bethemostefficient. Withtheporous-stone
sparger aboutthe same aeration was obtained with an airflowof0.2 volume
of air per volume of liquid per minute as was obtained with the single-
orifice sparger using five times as much air, i.e., 1 volume per volume of
a Manufactured by Fisher & Porter, Hatboro, Pa., USA
78 E. B. CHAIN AND OTHERS
TABLE I. SPARGER AERATION WITHOUT STIRRING
Type of sparger *
Air pru-tn
throughput small ring small ring large ring porous-stone
(volume ofair sinple- (45-mm (45-mm (130-mm cyli-mm
(18-mm
per minute oriIce diameter diameter diameter
persvoluome olf utiwoith2n7 hol)es) dwithi90homles)twitrh76holes) diameter)
oxygen absorbed (ml per 100 ml of solution per hour)
1.0 24 13 13 5 40
0.2 0 - - -21
* Spargers mounted centrally 50 mm from bottom of reaction vessel; sparger holes, 1-mm
diameter
liquid per minute. When equal volumes of air per volume of liquid per
minute were passed through theporous-stone sparger, the aeration obtained
was double that obtained with the single-orifice sparger using the same
amount of air.
Mechanical agitation in the presence of baffle-plates greatly increased
the efficiency ofthe sparger aeration (see table II). There was no significant
difference between the efficiency of the smaller ring sparger with 27 or
90 holes and the single-orifice sparger, so that the number ofholes, within
the limits of 1 to 90, does not seem to have a very important influence on
the extent of aeration. There was, however, a pronounced difference
between the aeration efficiencies of the smaller and larger ring spargers.
The larger sparger was much less efficient for the reason that most of the
air coming out from its holes passed through the liquid along the walls of
TABLE II. SPARGER AERATION WITH STIRRING * IN FULLY BAFFLED SYSTEM
Type of sparger**
Air throaghput small ring small ring large ring
(volume of air single- (45-mm (45-mm (130-mm
per minute orifice diameter diameter diameter
with 27 holes) with 90 holes) with 76 holes)
solution)
oxygen absorbed (ml per 100 ml of solution per hour)
1.0 64 58 58 23
0.2 20 24 - 9
* Speed of stirring, 500 r.p.m.; disc turbine, total diameter55mm, disc diameter
37 mm, 8 vertical blades 14 mm long, 15 mm high (see fig. 5)
** Spargers mounted centrally 50 mm from bottom of vessel ; sparger holes,
1-mm diameter
STUDIES ON AERATION - I 79
the reaction vessel without being subjected to the dispersing action of the
mechanical agitation. It follows from these experiments that if a ring
sparger is to be used for aeration in a fully baffled and agitated fermenter,
it is important that its diameter should be about the same as that of the
propeller. Frequently ring spargers of too large a diameter are used in
fermenters. In one case the authors found it possible almost to double
the aeration in a 200-litre fermenter by doing nothing else but reduce the
diameter of the sparger of this fermenter (which was about the same as
the diameter of the fermenter) to that of the propeller.
FIR. 4. COMPARISON OF AERATION GIVEN WITH DIFFERENT PROPELLER*
SPEEDS BY POROUS-STONE AND SINGLE-ORIFICE SPARGERS
320 - Single-orifice 1 mm diam.
*"-.--- Porous-stone cylinder
280 C Without stirring
cx A Propeller speed 500 rp.m.
°~240 0 Propeller speed 900 r.p.m.
E~ .e0 Vo o
o
° 2001 ,,,,,@
E~ 1 60.'
120 0,
0 ~~~ ~ ~ ~~~~~~.
CZ 80-
0 0,2 04 0.6 0.8 1.0
Vol. ofairper minute
Vol.ofso/n
* For type of propeller, see table 11 and fig. 5
There is, however, no particular advantage in using for fermentation
tanks ring spargers in preference to single-orifice spargers. Ring spargers
have the disadvantage that they frequently become blocked by mycelium
developing inside the tubes during the fermentation. This is less likely to
occurin single-orifice spargers because ofthehighervelocity oftheairflow.
The porous sparger also always gave superior aeration to the single-
orifice or ring spargers in the mechanically agitated, fully baffled system
(fig. 4). Thus, with a relatively low air flow (0.2 volume per volume of
liquid per minute) and moderate speed ofagitation (about 500 revolutions
per minute (r.p.m.), consuming 0.4 watt per litre), the porous-stone sparger
gave about three times the amount of aeration achieved with the single-
80 E. B. CHAIN AND OTHERS
orifice sparger. With higher propeller speeds, the difference became less
pronounced.
However, in practice, the porous spargers have the same disadvantage
as the ring spargers in that they become blocked by growing mycelium;
they are, moreover, difficult to clean.
Aeration and power consumption with single-orifice sparger in fully baffled
system
At present the most commonly used system ofaeration in fermentation
technology is the introduction ofair through a sparger into a mechanically
agitated liquid inthe presence ofbaffle-plates. The authors havedetermined
the degree of aeration achieved as a function of the power consumed by
the mechanical agitation with propellers ofvarying diameters. The propel-
lers used were ofthe turbine type with eight blades; they were found to be
the most effective. In all experiments aeration was effected by means of a
single-orifice sparger, placed centrally below the propeller.
It is well known (e.g.,Rushton 3) that in a fully baffled system the power
absorption bytherotatingpropellerincreasesexponentially with the number
of revolutions and the diameter of the propeller blades, according to the
formula
P = Kp N2-3D3-5
where
K = constant
p = density
N = number of r.p.m.
D = diameter of propeller blade (mm)
The authors found that the absorption ofoxygen by the sulfite solution
also followed an exponential function in relation to the number of revolu-
tions ofthe propeller blades. When the values for oxygen absorption were
plotted against the power consumption (fig. 5) an approximately linear
relationship became apparent for propellers of the two larger diameter
sizes (i.e., of75 mmand 90mm) forairvelocities of1 volumeand0.5volume
of air per volume of liquid per minute over nearly the whole range of
oxygen absorption tested. For the smallest propeller, of 55-mm diameter,
linearity was approached when the air velocity was an equal volume of
air per volume of liquid per minute. For the propellers of intermediate
size, approximate linearity was found for all air velocities over a wide
range of oxygen uptake, i.e., the zone of practical interest (50-200 ml per
100 ml of liquid per hour); there was a slight flattening of the curve in
the regions of the highest oxygen absorption (200-300 ml per 100 ml of
liquid per hour).
These results indicate that, for propellers above a certaindiametersize,
all the energy put into the system for mechanical agitation is utilized for
STUDIES ON AERATION - I 81
aeration in direct proportion to the degree of turbulence created. For
propellers below a certain diameter, direct proportionality between power
input and aeration is obtained only up to a point corresponding to a
relatively low degree of turbulence which is produced in a limited central
zone of the reaction vessel within the sphere of influence of the rotating
propeller, and not uniformly throughout the liquid. An increase of the
power input above this point does not lead to a corresponding increase of
general turbulence and hence is wasted as far as aeration is concerned.
In the creation of turbulence in a liquid, the effect of the baffle-plates
is of paramount importance. The contribution to aeration efficiency
arising from the shearing action of the turbulent liquid on the air-bubbles
becomes evident if the oxygen uptake is measured after removal of the
baffle-plates. When this is done at stirrer speeds exceeding a certain value
(depending on the propeller diameter) a vortex develops. Under these
circumstances special conditions ofaeration prevail which will be discussed
in detail later. To study the effect ofbaffle-plates on aeration it is necessary
to reduce the stirrer speed to velocities at which no vortex develops. The
FIG. 5. RELATIONSHIP BETWEEN OXYGEN ABSORBED AND POWER CONSUMED
IN FULLY BAFFLED SYSTEM WITH PROPELLERS OF DIFFERENT DIAMETERS
__d.sctwbrne5Owdive BbE fSmAigAhu=O4S
300 -T-b!Ctw.!rfsMe -- oxfvrieh*?$AMks ijwe710 k a43?5
di 70-0 w .S.- .*N .Q2
Vk9*X*-UK1~ Vt5L*UE X d5t ,<s
250 *.O5wLrp0.sd
1 ,
100~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~0d
SO :;P,'t-'- 551D_ _a-.'-0 wto
W_rieOFtFL. ,F , ' i ww
-&,Mo-EW - , .-;
v M W
IO 0. 10 1O5 .2.0 2.5 3n034S 42
_._.---
82 E. B. CHAIN AND OTHERS
TABLE 111. COMPARISON BETWEEN SPARGER AERATION * IN FULLY BAFFLED
AND UNBAFFLED SYSTEMS AT MODERATE PROPELLER SPEEDS
Air throughput with Air throughput without
baffles baffles
(volume of air per minute (volume of air per minute
Disc per volume of solution) per volume of solution)
turbine Propeller
propeller speed
diameter (r.p.m.) 0 0.5 1.0 0 0.5 1.0
(mm)
oxygen absorbed (ml per100 ml of solution perhour)
55 0 1 2
55 550 0 36 61 17 24
710 9 ill 133 23 52 62
65 400 27 - 60 6 12
75 p- 37 61 15 15 6
90 - 50 70 12 24
* Spargerwith single orifice, 1 mm in diameter, mounted centrally50 mm from bottom
of vessel
results of experiments conducted under these conditions are illustrated in
table It is evident that the aerationis less in the absence ofbaffle-plates.
III.
The power consumption is, of course, also reduced.
TABLE IV. EFFECT ON SPARGER* AERATION OF VARYING THE
NUMBER OF PROPELLERS ** ON ONE SHAFT IN FULLY BAFFLED
SYSTEM
Air throughput
(volume of air Number of Oxygen absorbed Power consumed
vopleurmemionfutsoelupteiron) propellers s(omllutpieorn 1p0e0r mhlouro)f (wpaetrt-lhitorue)rs
0.2 1 60 1
0.5
72
1.0 105
0.2 2 66 1.5
0.5 72 1.5
1.5
1.0 105
0.2 3 80 1.62
0.5 92 1.62
1.0 105 1.62
* Sparger with single orifice, 1 mm in diameter, mounted centrally 30 mm
from bottom of vessel
** Disc turbine, 55-mm diameter, with 8 vertical blades; speed 775 r.p.m.
It is frequently the custom in the fermentation industry to use stirrers
with several propellers placed on the axis at varying heights. The authors
haveinvestigatedtheeffect ofsuchsystems on aeration efficiency, comparing